It is known in the art to supply the operating chamber of the cylinder of the disc brake with vacuum to discharge air therefrom when charging of an operating fluid into the chamber is required. However, by the nature of the construction of the cylinder and pistons disposed in the cylinder, it is very difficult to completely discharge air from the operating chamber even when such vacuum supplying method is employed. Thus, it sometimes happens that air bubbles remain in the operating fluid in the operating chamber. Existence of such air bubbles in the chamber causes unstable and dangerous operation of the disc brake as is well known.